1. Field of the Invention
The present invention relates to an optical signal quality monitoring method and an optical signal quality monitoring apparatus that monitor the quality of an optical signal in an optical communication system and, in particular, to an optical signal quality monitoring apparatus and an optical signal quality monitoring method that monitor the quality of an optical signal by using a value estimated from a polarization state.
2. Description of the Related Art
With the expansion of communication capacities, optical communications at a transmission rate exceeding 10 Gb/s per optical signal channel have been put into commercial use and research and development of optical communications at a transmission rate exceeding 100 Gb/s is being actively conducted. In such an ultrahigh-speed optical communication system, even slight waveform degradation can significantly affect the quality of a signal received. Physical phenomena that cause waveform degradation include wavelength dispersion, polarization-mode dispersion, and nonlinear phenomena. Wavelength dispersion and nonlinear phenomena among these phenomena can be avoided to some extent by transmission path design and signal compensation techniques. However, there is no definitive way of avoiding polarization-mode dispersion at present. Therefore, great importance is being placed on accurate monitoring of polarization-mode dispersion and using the result for a control signal for dynamically compensating for waveform distortion or for a trigger signal for failure recovery.
For example, the following four methods for monitoring polarization-mode dispersion for compensating for waveform distortion have been proposed. A first method is to use the Degree of Polarization (DOP) as disclosed in J. C. Rasmussen et. al., “Automatic Compensation of Polarization-Mode Dispersion for 40 Gb/s Transmission Systems,” p. 2101, No. 12, Vol. 20, IEEE JLT, 2002. A second method is to use eye opening as disclosed in Zhihong Li et. al., “Chromatic Dispersion and Polarization-Mode Dispersion Monitoring for RZ-DPSK Signals Based on Asynchronous Amplitude-Histogram Evaluation,” p. 2859, No. 7, Vol. 24, IEEE JLT, 2006. A third method is to use an RF clock intensity equal to B/2 or a multiple of the symbol rate B of an optical signal as disclosed in JP2000-330079A. A fourth method is to use a State of Polarization (SOP) as disclosed in JP2004-138615A. It is known that these methods can monitor Differential Group Delay (DGD), which is the first-order component of polarization-mode dispersion.
There is also a method for evaluating the quality of a transmitted optical signal by evaluating a Q value as an evaluation measure.
WO2003/028254 discloses a method for calculating DGD based on a measured SOP. The method disclosed in WO2003/028254 uses measured SOP to calculate a Polarization Mode Dispersion (PMD) vector and DGD is obtained from the calculated PMD vector. Accordingly, when the method disclosed in WO2003/028254 is applied to an optical signal quality monitoring apparatus, the configuration of a section relating to the DGD monitoring of the optical signal quality monitoring apparatus cannot be simplified because the method requires the PMD vector to be calculated in order to obtain the DGD.
The techniques disclosed in the documents given above cannot accurately estimate a Q value if a high-order component of polarization-mode dispersion is not negligible. This is because, if a high-order component of polarization-mode dispersion is not negligible, the correlation between the DOP, eye opening, or RF clock intensity and the Q value will not hold. Therefore, it is difficult to apply the methods disclosed in J. C. Rasmussen et. al., “Automatic Compensation of Polarization-Mode Dispersion for 40 Gb/s Transmission Systems,” p. 2101, No. 12, Vol. 20, IEEE JLT, 2002 and Zhihong Li et. al., “Chromatic Dispersion and Polarization-Mode Dispersion Monitoring for RZ-DPSK Signals Based on Asynchronous Amplitude-Histogram Evaluation,” p. 2859, No. 7, Vol. 24, IEEE JLT, 2006 and JP2000-330079A to optical signal quality evaluation based on a Q value (Q value monitoring).
JP2004-138615A describes an example in which the first-order component of polarization-mode dispersion is obtained from the result of SOP monitoring and a first-order polarization-mode dispersion compensator is controlled to remove polarization-mode dispersion impairment. While JP2004-138615A makes no reference to the relationship between a first-order component of polarization-mode dispersion and a Q value, it is known that the Q value generally is inversely proportional to the first-order component of polarization-mode dispersion.
However, when applied to Q-value monitoring, the accuracy of the monitoring by the method disclosed in JP2004-138615A is low. This is because the method does not take into consideration a high-order component of polarization-mode dispersion. When a high-order component of polarization-mode dispersion is negligible, the correlation between SOP and the Q value is constant. However, when a high-order component of polarization-mode dispersion is not negligible, the correlation between SOP and a Q value varies depending on the magnitude of the first-order component of polarization-mode dispersion and therefore is not constant. Assuming that the correlation between SOP and Q value is constant when a high-order component of polarization-mode dispersion is not negligible, a large error is introduced in estimating a Q value from an SOP. Therefore, in order to accurately estimate a Q value from an SOP when a high-order component of polarization-mode dispersion is not negligible, the correlation between SOP and Q value needs to be corrected based on the magnitude of the first-order component of the polarization-mode dispersion.
Using a Q value having a large estimation error makes optical signal quality information imprecise. As a result, network control, such as failure recovery, based on the optical signal quality information can become difficult and the reliability of the optical network can degrade.